The present invention relates to spectrum analzyers, and more particularly to spectrum analyzers with the capability of displaying spectrum information over time.
A display of spectrum information over time is often referred to as a “spectrogram.” In a spectrogram frequency is along one axis and time is along another. Spectral amplitude is often denoted with color or with brightness gradations (grey scale). Several current spectrum analyzers on the market have a spectrogram display function including the Tektronix WCA200, WCA300, RSA2200 and RSA3300 series realtime spectrum analyzers. Such a spectrogram is also illustrated in FIG. 2 of U.S. Pat. No. 4,870,348. The above-referenced realtime spectrum analyzers capture seamless blocks of data for later analysis. Unlike conventional swept spectrum analyzers, no data within the blocks is missed or lost. One function that may be performed on this data is a function called a frequency mask trigger. In the case of the WCA200 this trigger is implemented with a dedicated fast Fourier transform (FFT) processor that works on the realtime data provided by its receiver system. When the user-defined frequency mask is violated, a predefined block of data is stored that includes information on what happened around the event that violated the frequency mask—data before the event being referred to as pre-trigger data and data after the event being referred to as post-trigger data. The frequency mask trigger works by comparing each spectrum the FFT processor calculates to a predefined mask. In this manner the spectrum analyzer waits for a specific single event to occur.
There are signals that occur where a carrier might be following an expected trajectory or have a particular sequence of frequency changes, such as is displayed by a hopping signal. A single event frequency mask is not able to provide a trigger that is a function of such complex events, but rather would provide inappropriate triggers as the carrier or hopping trigger changed frequencies.
What is desired is a frequency mask trigger that allows a spectrum analyzer to trigger on multiple or complex events.